This invention relates to supporting an elongate element from a surface vessel during an offshore operation. An example of such an element is a subsea pipeline that hangs as a catenary from a surface vessel toward the seabed during installation. In the art of subsea operations, supporting such an element may be referred to as ‘holding back’ or ‘hanging off’.
The main methods for installing subsea pipelines are known in the art as reel-lay, S-lay and J-lay.
In reel-lay operations, a pipeline of rigid or flexible pipe is initially spooled onto and stored on a reel on an installation vessel, which visits a coastal spoolbase at which the pipeline is fabricated. During installation offshore, the pipeline is unspooled from the reel and then overboarded into the sea to hang from the vessel as a catenary. Between unspooling and overboarding, the pipeline may pass through various types of equipment, such as tensioners, a tower, a ramp, a stinger or—if the pipeline is of rigid pipe—a straightener. The main hold-back forces are applied to the pipeline by the tensioners and the reel.
In S-lay operations, the pipeline is assembled from pipe joints along a horizontal firing line on an installation vessel offshore. As it is assembled, the pipeline is launched into the sea over a laying ramp or stinger to hang from the vessel as a catenary. The pipeline assumes an S-shape comprising an overbend over the stinger and an opposed sagbend approaching the seabed. Again, the catenary is held back by tensioners on the installation vessel.
In J-lay operations, in contrast, the pipeline is assembled from pipe joints in an upright tower on an installation vessel offshore. The pipeline hangs near-vertically to a sagbend approaching the seabed, thus assuming a J-shape. In this case, the catenary is held back by friction clamps or by a collar arrangement that is co-operable with bushings on the installation vessel. As friction clamps and collar/bushing arrangements are both static relative to the pipeline, this allows the pipelaying equipment of J-lay to be more compact and less massive than the tensioners that typify reel-lay and S-lay.
Pipeline installation in deep water requires the installation vessel to have a high hold-back capacity. This is due to the great weight of the long catenary of pipeline that is suspended between the vessel and the seabed.
The hold-back capacity of tensioners is limited because their endless-loop architecture limits the transverse pressure and consequently the longitudinal friction forces they can apply to a pipeline. For heavy pipelines such as large-diameter rigid pipelines, the weight of the catenary may be great enough to overcome the friction capacity of tensioners that are available to hold the pipeline. Consequently, the J-lay method is favoured for use where the water depth is great (for example, more than about 1000 m) and the pipeline is heavy.
At least two friction clamps or bushings are needed to lower a pipeline in J-lay operations. One of those clamps or bushings is movable reciprocally relative to the J-lay tower in opposed directions parallel to the lay direction to hold back and lower the pipeline in a hand-over-hand arrangement. For example, WO 2010/059035 features a combination of several collars and bushings.
Friction clamps are specifically designed to maximise frictional hold-back forces at the interface with the outer surface of the pipeline. Examples are disclosed in GB 2370335, WO 01/35011, US 2014/334879 and WO 2009/153354. Pads of the friction clamp hold the pipeline with friction generated by radially-inward squeezing force. The pads may have a special design for increasing the surface area of contact. For example, pads of the friction clamp disclosed in US 2014/334879 include protrusions that embed into the surface of the pipeline, to enhance frictional engagement by increasing a contact area between the clamp and the pipe.
In more distant prior art, US 2014/079486 discloses a friction clamp for gripping an umbilical having a smooth outer surface relative to another elongate element such as a pipeline.
The main drawback of friction clamps is their total reliance upon friction. This is disadvantageous because there is nothing to hold the pipeline if it starts to slip through the clamp, for example because the outer surface of the pipeline has a poor surface finish or is wet or oily.
In the alternative of a collar arrangement, also known as a J-lay collar, the collar is a metallic part of the pipeline that defines a radially-projecting ring. Examples are a forged radially-projecting ring welded to the pipeline, or a forging comprising such a ring that is welded to an end of a pipe joint of the pipeline. The collar mechanically engages a hold-back bushing on the pipelaying vessel, thus providing a steady and reliable mechanical connection between the laying equipment and the pipeline. For example, WO 99/01638 discloses collar flanges, whereas WO 2009/083937 discloses a pipe-in-pipe structure with several J-lay collars on the same pipe. US 2011/0226373 and U.S. Pat. No. 6,334,739 provide further examples of J-lay collars, and a J-lay collar is also illustrated in FIGS. 1 and 2, which will be described later.
J-lay collars have various drawbacks, including the cost of the specific forged pieces and the time and complexity of welding them into the pipeline. For example, the area around the interface between a collar and a pipe joint has to be bare steel to allow welding. Also, welding requires specific welding processes and qualifications because the metallurgical quality of the steels to be welded together is not homogeneous. In particular, a seamless pipe joint is made of an extruded billet of carbon steel whereas a forged collar has other phases in its metallurgical composition, being more ferritic or pearlitic.
After welding, a thermal insulation coating has to be applied to cover the full pipe section including the collars. Coating is performed in a work station that is situated beneath the hold-back bushing and so is beneath the welding station, which is risky for offshore crew. The quality and evenness of the thermal insulation coating around the radially-projecting ring of the collar is also a concern: a thermoplastic coating may not bond sufficiently with the discontinuous shape of the steel collar.
Effective thermal insulation is an important requirement for many subsea pipelines, especially those used to transport crude oil or natural gas from subsea wellheads. Oil and gas are present in subterranean formations at elevated temperature and pressure, which may be increased by the injection of fluids such as steam. As collected at the outlet of a wellhead, crude oil is a viscous, multiphasic, pressurised fluid whose temperature is typically around 100° C. to 180° C. but may be higher. If the crude oil is allowed to cool too much, some components of the oil may solidify by mechanisms such as coalescence, precipitation or gelling. The waxes, asphaltenes, hydrates or other solid condensates that appear as a result may form a plug that will clog the pipeline and be difficult to remove. Similar issues may arise in subsea pipelines used to transport natural gas.
Thus, during transportation along the pipeline, the temperature and pressure of the produced fluid have to be kept high enough to ensure a sufficient flow rate. In particular, various measures are taken to ensure that the internal temperature of the pipeline remains high, typically above 65° C. and in some cases above 200° C., despite thermal exchange with seawater which, for example, is at 4° C. below 1000 m depth.
In addition, an oil or gas field must occasionally be shut down for maintenance. During shut-down, production is stopped and so no hot fluid flows through the pipeline. Consequently, to avoid clogging by solid-phase materials, mitigating fluid such as methanol or diesel oil is injected into the pipeline during shut-down. When production restarts, temperature within the pipeline must be increased quickly so that no plugs will form.
It is important to maintain thermal management continuously along the length of a pipeline. Otherwise, ‘cold spots’ will arise, which increases the likelihood of plugs forming at those locations. J-lay collars increase the risk of cold spots.
Against this background, the invention addresses the conflicting challenges of providing a hold-back system for use in J-lay operations that is inexpensive, reliable and safe for offshore personnel and yet does not jeopardise thermal insulation of a pipeline.